1. Field of the Invention
The present invention relates generally to an improvement in an image forming apparatus. More particularly, it relates to a document sheet position and/or size detecting device for such an image forming apparatus.
2. Discussion of the Background
A variety of image forming apparatuses, such as digital copiers and facsimile apparatuses, have been developed and applied for practical uses, wherein images on an original document sheet are read by optical means such as line sensors of a CCD system or other types. The optical image information is then converted into electric signals which are fed to a printer or a similar recording apparatus to reproduce the images. In the operation of such an image forming apparatus, the initial step is to place the original document sheet in situ and to detect the position of the document sheet to prepare for initiating the scanning operation. For this purpose, a document sheet position detecting device is necessarily included in the image forming apparatus.
For example, Japanese Laid-Open patent publication (Kokai) No. 56-39555 (Toshiba) discloses a method of detecting the size of a document sheet positioned on an analog copier, wherein the document sheet is covered by a document cover having a spectral reflectance distribution which is different from that of the document sheet. The document sheet and the document cover are exposed to a light and the reflected lights are passed to a photo-coupler which is sensitive to the light reflected by the document sheet and insensitive to the light reflected by the document cover. However, in the system used in this known method, the same photo-coupler is used both for the normal image scanning operation and the operation for detecting the document size. This leads to the disadvantage that a filter must be provided to differentiate the sensitivity in the step of detecting the document size from the sensitivity during the normal image scanning operation. Another proposal has been made and disclosed in Japanese Laid-Open patent publication (Kokai) No. 56-22424 (Konishiroku), wherein a colored document cover is used and the light reflected by the document cover is received by a sensor so that the size of a document sheet is determined. However, this device is low in accuracy, since the white background portion of the document sheet has a wide spectral reflectance distribution and thus only a portion of the light reflected by the document sheet is received by the sensor to lower the accuracy in the document size detection operation. Japanese Laid-Open patent publication (Kokai) No. 59-225669 (Canon) discloses a device wherein the document sheet is covered by a member having a high regular reflectance, and sensor means is provided for receiving light rays which have been irregularly reflected by the document sheet. The area covered by the document sheet is determined by sensing the irregularly reflected light rays. However, this known device has a disadvantage that an undesired blurred fringe is formed around the document area on the reproduced copy when the size of the document sheet is smaller than the sheet on which the reproduced images are recorded or the document sheet is placed in the obliquely dislocated condition.